Method of fabricating clad tubes



Feb. 23, 1965 w. w. RICHMOND ETAL METHOD OF FABRICATING cum TusEsbOriginal Filed June 27, 1960 2 Sheets-Sheet 1 INVENTORII Ernest MontiusBYWilliomWRichmond 1965 w. w. RICHMOND ETAL 3,170,227

METHOD OF FABRICATING GLAD TUBES Original Filed June 27, 1960 2Sheets-Sheet 2 f i i i \V, H r H mm m\ fi h/ mw MN mm y m6 m Kw W 8 w 68 nulum l I i E INVENTORS Ernest Mcnrjus William W. Rlchmond BY M1ATTORNEY United States Patent The present application is a division ofUS. patent application Serial No. 38,992, filed June 27, 1960, nowabandoned. r

This invention pertains to the construction of heating tubes useful inacid concentrators and the like and is directed more particularly tocertain improvements there iri'for permitting greater'efiiciency in theoperation of the associated concentrator or other apparatus, while atthe same time imparting longer life to the heating tube itself. For thesake of brevity the description herein will be limited to heating tubesas constructed and employed in acid concentrators. It is to berecognized,however, that the invention as hereinafter disclosed andclaimed is equally applicable to other liquids and forms ofequipmentsuch as heating and cooling pots, open kettles, other forms ofevaporators, etc.

Certain types of acid concentratorsutilize heating tubes whichrareprojected into the interior of the concentrator containing the acid.Naturally, for greater thermal eiiiciency, it is desirable to use areasonably high steam pres sure and, consequently, high temperature inthe tubes to heatthe acid. However, certain limitations are imposed inview of the fact that'the heating tubes are directly in contact with theacid and hence are subject to considerable corrosion. In the past,attempts to overcome the corrosion effect have, unfortunately,introduced other factors substantially as detrimental as the corrosionitself. For example, high silicon iron tubes have been used Feb. 3, tastest. No.

ice j the like A maximum allowable steampressure with a.

tantalum tube of this type is usually about 150 psi. higher steampressuie's being possible by increasing the. wall thickness butfbeinguneconomical due to the high'co'st of the tantalum. i

In the present invention, a heating tube isformed from is core of mildsteel or other suitable material having sufficient strength to withstandsteam pressuresof 300 psi.

or higher and which core is suiiiciently strong to withstand thermalshock as well as mechanical shock and wherein there is intimately bondedto the outer surface of this core a thin Walledtantalum. tube whichrenders the tube corrosion resistant and anti-fouling while retaininggood heat transfer characteristics.

While this description of the invention is primarily concerned with theuse of a mild steel core, 'itsho'uld be understood that the core tubemay be made of other materials for example copper, nickel, titanium,etc. The employment of mild steel, however, is obviously economical andin most instances desirable as is hereinafter more fully set forth.

More particularly, the present invention contemplates with a metallicbonding medium such as for example a brazing compound to a suitablethickness, say about 0.001

to about 0.006 inch whereaftcr a preformed tantalum tube (14.4% Si,balance essentially Fe) and although this type cal damage in handling orthermal shock in use. Furthermore, this type of tube is not of unusuallygreat strength and maximum steam pressure is limited for practicalpurposes to about 150 psi. (365 F.). Corrosion is low but considerablefouling occurs and repair of broken tubes is not practical and when oncedamaged, a tube is completely lost. Still further, in view of the factthat the material is not of great strength, heavy wall thicknesses arerequired, in the order of three-fourths-of an inch, which, of course, isdetrimental to good heat transfer characteristics. Other alloys havebeen used, notably alloys of the. type containing 85% Ni, 10% Si, withthe balance substantially Cu, Al, etc., tubes made from such alloysbeing less brittle than the high silicon iron type and also beingcharacterized by permitting some repairs to be made in the case of tubedamage. However, such tubes are subject to considerably more corrosionthan the high silicon iron type. Steam pressure used with such type oftubes is usually in the range of about 300 p.s.i. (420 F.)

so that the amount of heat transferred is improved. In

'to the expense of the material involved, the wall thicknesses arenecessarily kept rather small, in the order of .015 inch and for thisreason, damage to the tubes easily Also, tubes 1 occurs from mishandlingor mechanical rupturing and is slipped over this core and with thetantalum tube and steel core being rigidly held together in a specialfixture is furnace brazed to effect a good bond between the tantalumtube and the mild steel core. The tantalum tube is provided with aflange mating with a corresponding fiange on the steel core and by meansof which the two tubes are rigidly held together during the furnacebrazing operation and therefurther being means to exhaust air frombetween the tantalum tube and the steel tubular core while the same arebeing furnace brazed.

Perhaps at this point it should be emphasized that a high degree ofvacuum is required to insure'the minimum presence of oxygen. At thebrazing temperatures herein contemplated the tantalum will'tend to beoxidized unless adequate provisionsto prevent same are maintained. Of

course replacement of any oxygen with an inert gas is helpfulin thisrespect. a

A further object of this invention resides in the method of fabricatinga clad tube in which tantalum sheet is Wrapped about a tube section andwelded into cylindrical form while Wrapped about the section, thetantalum sheet then being removed from the section and a metallicbonding medium such as copper or copper alloy inthe order of 0.001 to0.006 inch in thickness being applied to the tube, the tantalum sheetthen being re-assembled over the tube section and thetwo inserted in avacuum furnace at a temperature in the order of 2000 F. to .efiectabond. In this manner, the bond between the tantalum and-the steel tubesection is very effectively achieved by' virtue of the factthat thecoefficient of thermal expansion for steel is nearly twice that fortantalum so that; as the two are heated in the brazing furnace, thetantalum will be held tightly in compression on the steel tube section,the brazing material solidifying, during the cooling period, while thetantalum is still .in compression on the' steel, thus effecting a veryefficient bondbetween the two. As mentioned before at such furnacetemperatures it is important to keep air or oxygen away from thetantalum,

In the drawings:

FIG. 1 is asection, taken. through a portion of atypicalacidconcentrator, showing several tubesoperatively positione d therein;

anazzi FIG. 3 is an end elevational view of the assembly shown in FIG.2;

FIG. 4 is an enlarged section taken through a portion of the heatingtube and illustrating the manner in which the tantalum cladding is heldin place on the tube on the steel core during the furnace brazingoperation;

FIG. 5 is a plan view showing a modified method of cladding andillustrating the initial step thereof; and

FIG. 6 is a longitudinal section taken through one of the tubeassemblies fabricated in accordance with the modified method.

Referring now more particularly to FIG. 1, a portion of the acidconcentrator is indicated therein generally by the reference character10 and will be seen to include inner wall portions 11 and 12 and anouter wall 13 provided with openings therein through which project aplurality of heating tubes such as those indicated by the referencecharacters 14 and 15. In the operation of the concentrator, the diluteacid is introduced into the concentrator and covers the tubes 14 and 15and these tubes are heated by means of pressurized steam or otherheating media so as to heat the dilute acid during the concentratingprocess until such time as a predetermined acid concentration has beenachieved, as necessitated by its ultimate use.

As shown more clearly in FIG. 2, a heating tube constructed inaccordance with this invention typically con-.

sists of a mild steel tubular core indicated generally by the referencecharacter 16 and being constructed preferably of an elongated tubularlength 17 of such material and being provided with caps 18 and 19 at itsopposite ends which are aifixed and sealed to the main section 17 bymeans of the welds 20 and 21. A steam or other heat containing mediainlet line 22 is projected into the interior of the core 16 through theend cap 18, the inlet tube being rigidly fixed to the core by means ofthe weld 23. The inlet tube 22 is provided with a flange 24 at its outerend so that the same may be detachably secured to a source of steamunder pressure. For returning the steam condensate, there is acondensate line 25 projecting angularly downwardly from the bottom ofthe core 16 and being provided with a securing flange 26 at its free endas shown for connection to a condensate return line in the steam supplysystem.

Fixed to the main section 17 of the core 16 adjacent the cap 18 is anannular collar 30 which is rigidly secured to the core by means of theweld 31 and which collar, as can be seen best in FIG. 3, is providedwith a plurality of circumfernetially spaced openings 32 whereby thetube assembly is detachably secured in proper position in associationwith a concentrator such as that indicated by the reference character 10in FIG. 1.

Surrounding the steel core 16 is a tantalum tube 33 which is providedwith a flange 34 at one end abutting the collar 30 and having a raisedcircumferential rib 35 thereon projecting into a correspondingcircumferential groove in the face 36 of collar 30, effecting a sealbetween the flange 34 and collar 30.

The tantalum tube 33 is bonded firmly and securely to the steel core 16by means of an intermediatelayer of brazing compound 37. Preferably, thetantalum tube is formed from sheet stock and is relatively thin,preferably in the order of 0008-0015 inch in thickness. The bondingmaterial 37 is, on the other hand, of a thickness in the order'of about0.001 to about 0.006 inch.

In manufacturing the tubes, the steel core assembly, after fabrication,typically has a layer of brazing compound sprayed or otherwise appliedover the surface thereof beyond the collar 30 and then the preformedtantalum tube and flange are slipped over the core to bring the flange34 thereof up against the collar 30. At this point, an annular collar40, see FIG. 4, is positioned against the flange 34 and securedclampingly to the collar 30 by means of a series of studs or bolts 41which pass through suitable openings 42 in the collar and the openings32 in the collar 30, substantially as shown. From FIG. 4, it will alsobe noted that at least one point in the collar 30, there is provided anexhaust port 43 which projects through a collar 30 and may be internallythreaded in one portion 44 thereof for connection to an exhaust line.When the clamping collar 40 is in place as shown in FIG. 4, the assemblyis introduced into a brazing furnace where the temperature of theassembly is typically elevated to the order of 2000 F. for example whencopper is employed, for a period of time so that the brazin compound mayfuse and firmly unite the tantalum tube 33 to the core 16. At the sametime, a vacuum is maintained in the exhaust port 43 to exhaust any airor fumes accumulating between the steel tube and the tantalum tubeduring the brazing operation. After the brazing operation, the tubeassembly is permitted to cool and clamping collar 40 is removed and thenet result is a tantalum clad mild steel tube having material strengthwithout being brittle and having, by virtue of its tantalum cladding, ahigh resistance to corrosion as well as fouling and while stillretaining excellent heat transfer characteristics.

The rib 35 and the cooperating groove in the portion 30 which receivessuch rib as well as the rib on the collar 40 which serves to hold therib 35 in place cooperate together to form an air-tight seal extendingcircumferentially completely around the flange 34 as will be clear fromFIG. 4 of the drawings so as to provide a seal in this region. In thisway, it will be assured that there will be no leakage exteriorly pastthis rib when negative pressure is introduced through the port 43 andhence the negative pressure thus introduced will be fully elfective toassure the withdrawal of air, fumes and other gaseous products duringthe furnace brazing operation. This is an important considerationinasmuch as it is essential to provide as good a bond between the tube33 and the core which it surrounds as is possible in order that the heattransfer characteristics will be of the best obtainable.

It is to be noted that by the specific process and assembly disclosedherein, a very thin tantalum tube is utilized. When using tantalumtubing, without the metallic core, the thickness of the tantalum must beincreased as the diameter of the tube and/or the internal steam pressureis increased, whereas in our specific process the thickness of thetantalum remains the same under all conditions. Since the cost oftantalum sheet from which the tubes are normally fabricated is about$70.00 per pound, it will be readily appreciated that the instantprocess and construction represents a material saving in cost while atthe same time, the instant assembly is also able to withstand muchgreater steam pressures than tantalum tubing alone and is not subject toas great a degree of mechanical damage as the tantalum tubing alone.

Referring at this time more particularly to FIG. 5, a modified method offabrication is illustrated. In this figure, reference character 60illustrates a steel tube section which is utilized in this instance as awelding jig. This tube section Operates to act as a form about which asheet 61 of tantalum is wrapped, with the longitudinal edges thereof inabutting or slightly overlapping relation and while held firmly aroundthis tube, such longitudinal edges of the sheet are welded together asindicated by the reference character 62 to effect a cylinder of tantalumsheet wrapped about the tube section 60. During this operation, the endcap indicated generally by the reference character 63 is also held onthe steel cap 65 and it likewise is welded to the sheet 61 as indicatedby the reference character 64 in FIG. 6.

The tube section 60 may be either the tuge section to which the cylinderformed by the sheet 61 and end cap 63, as shown in FIG. 5, will beultimately bonded, or it may be a special tube section used for thispurpose only. In the former case, the cylinder formed by the sheet 61and the endpcapi63 is removed from the tube section 69 and then thistube section 60 is coated with 'say from .002 to .005 inch of coppeia,The two tubes, that is the tube section 60 with copper plating thereonand the tantalum cylinder or tube comprising the'welded sheet 61 and the'endcap -63 are then re-assembled and inserted in a vacuum furnace at abrazing temperatureof about are then allowedto cool to room tempera- Ving the greater coefiicient of thermal expansion, 'will'ex pand to adegree such as to effect a-substantial degree of tension in the outertantalum tube when the two are sub ,jected to the brazing temperature."Naturally, care must betaken that the tantalum is not stretched beyondits elastic limit. From this, it will be apparent that the tantalumsheet will be very firmly compressed upon the steel tube at the brazingtemperature and asubstantial degree of this compression will be presentduring'the initial cooling step, at which time the brazing compoundsolidifies so as to, therefore, effect a complete metal-to-rnetal :bondat the interface of the two tubes or cylinders.

Alternatively, the tube '60, which "forms the welding jig for theformation of the tantalum cylincler or tube, may be a memberspecifically utilized for this purpose only. In such case, it is, ofcourse, essential that the tube 60 so used be accurately dimensioned andthat the tube ultimately brazed to the tantalum cylinders formed thereonbe of accuratedimensions so that the aforementioned' metal-to-metalbonding at the interface between the two sheets be effected.

. The finished product is illustrated in FIG. 6. in this figure, by wayof specific example, the tube section 60 may be a five inch extra heavycarbon. steel pipe having a cap 65 Welded tonne end thereof as indicated'by the reference character 66. The. tantalumsheet 61'is of the order of0008-0015 inch in thickness and the end cap 63 is of like thickness andmay be spun or drawn to the desired shape, After the sheet 61 and endcap 63 are brazed to the tube section 60, the flanges 67 and 68 areadded. The flange 67 is of steel and is Welded as at 69 directly to thetube 60, whereas the flange 68 is of tantalum of 0008-0015 inchthickness having a band portion 70 circumferentiallywelded as at .71directly to-the tantalum sheet 61.

Thereafter, the assembly indicated generally by the reference character72 is welded as at 73,to the tube sectitan-60, thus completing thefabrication of theassemblage. The assembly 72 is prefabricatedpreferably so that the entire assemblage thereof is preformed and readytobe welded at 73 to the associated tube 60.

The steel inlet tube 74-, which forms part of the assemblage 72, may besupported adjacent its open end 75 by means of an apertured plate 76welded as at 77 to the inside of the tube section 60 and affixed thereinbefore the cap 65 is welded thereto.

The method as set forth hereinabove in relation to FIGS. and 6 lendsitself quite readily to semi-mass production. For example, the method,it will be appreelated, permits the steam head assembly 72 to be pre-.

fabricated separately and, as mentioned hereinabove,a dummy steel tubemay be employed as a welding jig for the tantalum sleeves and attachedend caps. In this manner, the fabrication of the individual assemblagesmay be effected more rapidly so long as the proper tolerances aremaintained throughout the production run.

As this invention may be embodied in several forms Without departingfrom the spirit or essential characteristics thereof, the presentembodiment is therefore illustrative and not restrictive, and since thescope of the inthat fall within the metes and bounds of the claims orthatYform their functional as well as 'conjointly cooperae tiveequivalents are therefore intended to be embraced by those claims. t

We claim: a 1

1. The method of fabricating heating tubesuseful for .acid concentratorswhich comprises first forming a' closed hollow mild steel elongatetubular member, forming an inlet and an outlet in said member at one endthereof,

applyingbrazing compound upon a portion of the outer surface of saidmember including one outer. end surface thereof remote from said inletand said outlet, inserting fthecoated end of the member into athinwalled tantalum let and an outlet in said member at one end thereof,applying brazing compoundupon a portion ofthe outer surface of saidmember including one outer end surface thereof remote from-said inletand said outlet, inserting the coated end of the member'into athin-Walled tantalum tubular envelope closed at one end thereof, heatingthe envelope and member so as'to effect a tension upon the envelope 'todraw the same onto the member and maintaining the heat at sufficienttemperature as to fuse the brazing compound and thus .join the envelopeand member, and simultaneously with the heating of the envelope andmember maintaining a vacuum between the envelope f and member towithdraw gasesfrom therebetween.

3. The method of fabricating heating tubes useful for acid concentratorswhich includes the steps of wrapping tantalum sheet about a cylindricalform and welding the longitudinaledges of the sheet together to form asleeve,

and simultaneously providing an end cap'for such sleeve,

' diameter of the tantalum sleeve, assembling the tantalum vention isdefined by the appended claims, all changes 7 I sleeve upon the coatedsteel sleeve and then subjecting the thus assembled sleeves to a brazingtemperature of about 2000 F. and then permitting the same to cool toroom temperature to effect a metal-to-metal bond at the interface of thetwo sleeves.

4. The method of fabricating heating tubes useful for acid concentratorswhich comprises welding an end cap on 'a section of steel tube to form asteel sleeve closed at one 'end, wrapping tantalumsheet aboutthesteelsleeve and over the major longitudinal extent thereof and fitting atantalum end cap over the end of the steel sleeve into contactingrelationship with the tantalum sheet and with the tantalum sheet havingits longitudinal edges disposed closely adjacent each other, welding thelongitudinal edges of the tantalum sheet together and welding thetantalum end cap'to the tantalum sheet to form a tantalum sleeve closedat one end, Withdrawing thetantalum sleeve from the steel sleeve,coating the steel sleeve with a brazing compound to a plate thickness ofabout 0.001 to 0.006 inch, re-assembling the tantalum sleeve upon thesteel sleeve, heating the i e-assembled sleeves to a temperature ofapproximately 2000 F. to expand the steel sleeve within' the tantalumsleeve so that the tantalum sleeve effects a compressive force upon thesteel sleeve and to liquefy the brazing compound on the steel sleeve,and then permitting the heated sleeves to cool to room temperaturewhereby the liquefied brazing compound solidifies while the tantalumsleeve is still effecting a compressive force upon the steel sleeve. 7

5. The method of fabricating heating tubes useful for acid concentratorswhich comprises welding an end cap on a section of steel tube to form asteel sleeve closed at one end, wrapping tantalum sheet about the steelsleeve and over the major longitudinal extent thereof and fitting atantalum end cap over the end of the steel sleeve into contactingrelationship with the tantalum sheet and with the tantalum sheet havingits longitudinal edges disposed closely adjacent each other, welding thelongitudinal edges of the tantalum sheet together and welding thetantalum end cap to the tantalum sheet to form a tantalum sleeve closedat one end, withdrawing the tantalum sleeve from the steel sleeve,coating the steel sleeve with copper to a plate thickness of about 0.001to 0.006 inch, re-assembling the tantalum sleeve upon the steel sleeve,heating the re-assembled sleeves to a temperature of approximately 2000F. to expand the steel sleeve within the tantalum sleeve so that thetantalum sleeve effects a compressive force upon the steel sleeve and toliquefy the copper plating on the steel sleeve, and then permitting theheated sleeves to cool to room temperature whereby the liquefied copperplating solidifies while the tantalum sleeve is still effecting acompressive force upon the steel sleeve, and thereafter affixingattaching flanges to the joined steel and tantalum sleeves.

6. The method of fabricating heating tubes useful for acid concentratorswhich comprises welding an end cap on a section of steel tube to form asteel sleeve closed at one end, wrapping tantalum sheet about the steelsleeve and over the major longitudinal extent thereof and fitting atantalum end cap over the end of the steel sleeve into contactingrelationship with the tantalum sheet and with 1 the tantalum sheethaving its longitudinal edges disposed closely adjacent each other,welding the longitudinal edges of the tantalum sheet together andwelding the tantalum end cap to the tantalum sheet to form a tantalumsleeve closed at one end, withdrawing the tantalum sleeve from the steelsleeve, copper plating the steel sleeve to a plate thickness of about0.001 to 0.006 inch, re-assembling the tantalum sleeve upon the steelsleeve, heating the re-assembled sleeves to a temperature ofapproximately 2000" F. to expand the steel sleeve within the tantalumsleeve so that the tantalum sleeve effects a compressive force upon thesteel sleeve and to liquefy the copper plating on the steel sleeve, andthen permitting the heated sleeves to cool to room temperature wherebythe liquefied copper plating solidifies while the tantalum sleeve isstill effecting a compressive force upon the steel sleeve, separatelyfabricating a steam head assemblage, and then affixing the separatelyprefabricated steam head assembly to the open end of the steel sleevehaving the tantalum sleeve joined thereto. i

7. The method of fabricating heating tubes useful for acid concentratorscomprising forming a closed hollow steel elongate tubular member,forming an inlet and an outlet in said member adjacent one end portionthereof, forming an annular collar on said member spaced from said oneend portion, applying a layer of brazing compound over the outer surfaceof said member beyond said collar and remote from said one end portionand over the opposite end portion of the member, providing a thinwalledtantalum tube closed at one end and having a flange at the opposite endthereof, fitting'said tantalum tube over said member and bringing saidflange up against said coller, positioning an annular collar againstsaid flange and clamping said last-mentioned collar to saidfirst-mentioned collar to provide a circumferentially extendingair-tight seal between said flange and said firstmentioned collar,applying a vacuum between said first mentioned collar and said flangeand inwardly of said airtight seal to exhaust gases accumulating betweensaid member and said tube, and simultaneously with the application ofsaid vacuum heating the tube and member so as to effect a tension uponthe tube to draw the same onto the member, and maintaining the heat at asufficient temperature as to fuse the brazing compound and thus join thetube and member.

References Cited by the Examiner UNITED STATES PATENTS 270,452 6/ 83McCormick 113-100 1,511,056 10/24 Ercanbrack -142 1,690,684 11/28Johnson 29-473.9 X 2,111,791 3/38 Larson 113-120 2,713,196 7/55 Brown29-198 X 3,032,316 5/62 Kramer 29156.8 X

FOREIGN PATENTS 15,023 1908 Great Britain.

WHITMORE A. WILTZ, Primary Examiner.

1. THE METHOD OF FABRICATING HEATING TUBES USEFUL FOR ACID CONCENTRATORSWHICH COMPRISES FIRST FORMING A CLOSED HOLLOW MILD STEEL ELONGATETUBULAR MEMBER, FORMING AN INLET AND AN OUTLET IN SAID MEMBER AT ONE ENDTHEREOF, APPLYING BRAZING COMPOUND UPON A PORTION OF THE OUTER SURFACEOF SAID MEMBER INCLUDING ONE OUTER END SURFACE THEREOF REMOTE FROM SAIDINLET AND SAID OUTLET, INSERTING THE COATED END OF THE MEMBER INTO ATHIN-WALLED TANTALUM